Why does mileage drop in winter?

Why does mileage drop in winter?

This week's Greenlings topic came to us from another reader tip. Don asked why his fuel economy suffers so much in winter weather. In his own experience, mileage drops about 10 percent when the temperatures go from the 60-70 F range to near freezing. This is consistent with our own experience and in fact we've seen even bigger drops than that when testing hybrid vehicles in winter conditions.

There are a number of reasons why winter driving reduces efficiency ranging from fuels to driver safety and comfort. If you happen to live somewhere like California or Florida where winter means night-time lows dipping into the sixties, you may not have experienced this phenomenon, but those of who live places with actual seasons have grown accustomed to this. Read to learn more about why your car uses more fuel when its cold.

Probably the best known reason for lower winter mileage is reformulated fuel. It's important to recall that there is actually no such chemical as gasoline. Gasoline is actually a blend of hydrocarbons along with other chemicals. Among the many standards established by ASTM (American Society for Testing and Materials) is pump gasoline. The standard specifies the properties that fuels must have but not the actual composition.

Over the years, refiners have modified the formula for gasoline in order to improve vehicle performance and drivability and reduce emissions. At normal ambient temperatures, gasoline will evaporate. The rate at which it evaporates is dependent on the composition. When petroleum is being refined a number of lighter and heavier components are produced and blended to create gasoline.

The lighter components evaporate more readily but have a lower freezing point. Similarly, the heavier bits are less prone to evaporation. The heavier components are used in so-called summer gas to make it easier to pass evaporative emissions standards and avoid issues like vapor lock. The lighter components make it easier to start the car in cold weather. A number of other components are added to winter gasoline to help lower the freezing point. However the combination of the lower density of the lighter components and the space taken up by additives means less overall energy per gallon when using winter formulas compared to summer formulas. As a result, the engine has to use more fuel to generate the same performance.

Cold temperatures also cause an increase in the viscosity of lubricants in the engine, transmission and axles all of which raise resistance to motion until they warm up.

Beyond fuel, there are other temperature related factors. While drivers looking for maximum efficiency are often willing to forego using air conditioning in the summer time, when the temperatures dip below freezing, just breathing inside the car causes the windows to fog up. That means it's absolutely necessary to run features like window defoggers which can put a significant drain on the car's electrical system.

Add in the fact that batteries tend to lose a lot of their power when they get cold, and it means the alternator has to work a lot harder on those cold mornings. If you have a car with other items like heated seats, it gets even worse. When we tested the Mercury Milan Hybrid in February, using the Smartgauge allowed us to see just how much energy the power accessories were consuming, particularly the defoggers. Factor in the effect on the battery of a hybrid and the situation gets even worse than for a conventional car. The hybrid is dependent on its battery and, when it is cold, the engine has to do even more work. Over a week of winter driving we averaged 28 mpg compared to 37 mpg for another Fusion hybrid that we tested during the summer over the same roads. We've seen similar results from other hybrids.

There are of course other factors outside the car that cause efficiency to drop. When roads are slick, tires slip more than they do on dry roads causing lost motion. Snow on the ground dramatically increases rolling resistance, making the car work harder to get motion. Even air resistance can increase somewhat as a result of increased density at low temperatures.

When plug-in vehicles arrive in the coming years, they will also take a performance hit in the dark season although automakers are working to mitigate this. Vehicles with a liquid cooled pack like the Chevy Volt may end up with an advantage because they can better regulate the temperature when plugged in. When ambient temperatures drop, the Volt battery will be pre-warmed while plugged in to ensure that it is at the optimum operating temperature when needed.

All of these factors and more mean that even the most thrifty drivers will see increased fuel consumption when the mercury drops. This is especially true for those that drive relatively short distances where the lubricants may not have an opportunity to full warm up or the driving distance beyond the warm up is short. Over longer trips, the warm-up period will be less of a disadvantage as it gets averaged out, but there will still be a penalty.